Detector track circuit for railroad crossings

ABSTRACT

A separate transmitter source of periodic high voltage pulses is coupled to each approach section rails at the end distant from the permanent shunt across the rails at a railroad crossing. Each pulse transmits a high level oscillating current pulse through the rail loop including the intersecting rail shunt so that similar track currents flow in both approach sections. A single receiver means is coupled to each rail shunt by a separate pickup coil, the output of which is rectified to charge a storage capacitor at an averaged voltage level representative of the amplitude of the current pulses in the corresponding shunt. The voltages stored in both capacitors are applied to the dual inputs of a two channel, vital level detector and logic AND circuit element. An output signal is produced by the level detector and AND circuits only when both inputs are at or above a level representative of an unoccupied section. The output is then applied, through a relay driver circuit, to a vital type track relay to jointly register the occupancy condition of both crossing approaches.

United States Patent [191 Grundy DETECTOR TRACK CIRCUIT FOR RAILROAD CROSSINGS Inventor: Reed H. Grundy, Murrysville, Pa.

Assignee: Westinghouse Air Brake Company,

Swissvale, Pa.

Filed: Dec. 8, 1972 Appl. No.: 313,608

US. Cl 246/114, 246/40, 331/166 Int. Cl B611 23/30 Field of Search 246/34 R, 34 CT, 40, 114;

References Cited UNITED STATES PATENTS 3/1949 Zarem et al. 331/166 5/1959 Leroy 246/34 R 6/1972 Sibley 246/40 8/1972 Crofts 246/40 Beday 1 June 25, 1974 [57] ABSTRACT A separate transmitter source of periodic high voltage pulses is coupled to each approach section rails at the end distant from the permanent shunt across the rails at a railroad crossing. Each pulse transmits a high level oscillating current pulse through the rail loop including the intersecting rail shunt so that similar track currents flow in both approach sections. A single receiver means is coupled to each rail shunt by a separate pickup coil, the output of which is rectified to charge a storage capacitor at an averaged voltage level representative of the amplitude of the current pulses in the corresponding shunt. The voltages stored in both capacitors are applied to the dual inputs of a two channel, vital level detector and logic AND circuit element. An output signal is produced by the level detector and AND circuits only when both inputs are at or above a level representative of an unoccupied section. The output is then applied, through a relay driver circuit, to a vital type track relay to jointly register the occupancy condition of both crossing approaches.

9 Claims, 2 Drawing Fi ures l Hecewer DETECTOR TRACK CIRCUIT FOR RAILROAD CROSSINGS My invention relates to detector track circuits for railroad crossings. More particularly, this invention relates to a detector track circuit usable for detecting trains in track sections which are permanently shunted at least at one end when one rail of an intersecting railroad track forms an electrical shunt across the rails of another track.

A major problem in the design of signaling systems to control the movement of trains through railroad interlockings, in which two or more tracks cross each other at grade, has been the furnishing of means to reliably detect the presence of a train or cars occupying either track in the immediate vicinity of the actual rail crossing. This problem arises because of the structural arrangement of the rails necessary to provide strength to resist rail shocks through the crossing area which re sults in permanent electrical shunts across the rails of each track by each rail of the intersecting track. The insulation of such rails is not possible at the crossing point due to the physical crossing structure and the necessity of assuring mechanical strength. Even closeto the crossing rail pieces, insulated joints create problems to install and are very difficult to maintain due to the heavy rail vibration caused by the wheels of trains moving through the crossing frogs. One method in general use in the past has been to detect the trains in the more distant approach sections by conventional track circuits terminating at insulated joints installed a reasonable distance from the crossing to reduce the amount of maintenance. The system then supplements these detector track circuits with a train check-in, check-out arrangement for the noninsulated portion of the crossing. Such combinations of track circuits and check-in. check-out arrangements are generally known as trap circuits. Several differenttrap circuit systems for detecting the movement of trains through such track arrangements are known. However, it is possible, and such situations have occurred. for a train to check in and check out properly through the noninsulated portion, thus clearing the trap circuit but to inadvertently leave a car in the noninsulated section where it is not detectable by the approach track circuits which terminate some distance away. Such an occurrence, of course, creates a possible accident situation. A standing car should obviously be visible but busy train crews, moving on. fail to notice the lost car. Than another train approaching along the intersecting track, having received a clear signal, cannot be stopped in time after the car becomes visible to the crew. Thus, a better method of detecting the presence of a train or cars occupying a railroad grade crossing is needed. There are also locations where it is difficult to adapt conventional track circuits to detect trains at all points in a switching area where a complex pattern of spur tracks turn out from one or more principal tracks. Because of the presence of permanent rail shunts in such tracks, a similar need thus arises for improved train detection arrangements.

Accordingly, an object of my invention is an improved detector track circuit for the track sections at railroad grade crossings. 4 v

Another object of the invention is a track circuit arrangement for detecting the presence of a car in a track 1 section having a permanent shunt across one or both ends;

A further object of the invention is a pulse type track circuit for track sections in the immediate approach to a railroad grade crossing with another track, which will detect the presence of a car within such approach sections.

Still another object of my invention is a high voltage, pulse type track circuit arrangement for detecting the presence of any part of a train occupying the track in the immediate approach on either side of a grade crossing of that track with a second railroad track.

It is also an object of this invention to provide a track circuit arrangement for detecting the presence of a car in a section of railroad track positioned between its intersection with two parallel tracks which cross at grade.

Another object of the invention is a track circuit for railroad grade crossing approach track sections with a pulse transmitter source connected to the rails at the approach end and a track receiver inductively coupled to the rails at the premanent rail shunt at the crossing for detecting the presence of a train or car in the entire approach section to the rail shunt.

An additional object is a high voltage, pulse type track. circuit for a separate track section having a permanent intersecting rail shunt across the rails at one or both ends, including transmitter and receiver apparatus coupled to the rails at selected locations to detect the presence of a train at any point within the section up to the permanent rail shunt or shunts.

A still further object of the invention is a detector track circuit arrangement which will assuredly detect the presence of a train or car approaching a railroad grade crossing up to the immediate point at which the rails of the second track cross the track occupied, and

also as the train recedes from the crossing with no gap in the detection which may be occupied by any part of a car without its presence being detected.

Other object, advantages, and features of my invention will become apparent from the following specification when taken in connection with the accompanying drawing and the appended claims.

In practicing the invention, a pulse transmitter source of track circuit energy is connected across the rails at the distant end of the approach track section to a railroad crossing, that is, the track section having the permanent shunt across its other end formed by the rail of the intersecting track. Track more distant from the crossing is isolated by insulated joints and normally provided with a conventional detector track circuit. In the arrangement herein, this pulse transmitter comprises a high voltage, direct current source connected across the rails in series with a resistor, to limit the maximum current flow, and a capacitor. In parallel with the source and between the source and the capacitor, a controlled rectifier is connected across the track leads poled to allow current from the source to flow when the controlled rectifier becomes conducting. The conducting condition of this controlled rectifier is conventionally controlled by a periodic gate signal source. Also connected in parallel across the track leads is a conventional diode oppositely poled. When the controlled rectifier is turned off, i.e., blocks current flow, the capacitor is charged to the voltage of the source, thetime constant of the charging circuit being relatively short. When the controlled rectifier is gated on and the charge on the capacitor is sufiicient, the rectifier conducts and voltage is applied to the rails from the capacitor, initially equivalent to the voltage of the high voltage source. Since the rails are of relatively short length, in this approach track circuit, a large circulating track current flows. Further, the inductance of the rails in series with the capacitance causes this loop current to oscillate or, in other terms, to ring. The oscillation period and the number of cycles are determined by the circuit parameters and the length of the gating signals applied to the controlled rectifier. The oscillating current is cut off when the controlled rectifier is no longer gated at the beginning of a positive or proper polarity half cycle. This high current level and the pulsating nature of the track energy cause a break through of any rust or film on the rails to improve the wheel and axle shunt. Since quite frequently at least one of the tracks included in a railroad crossing may have a very low frequency of train movement, the breakdown of rust or other film to improve the shunting characteristics is an important feature. A similar track transmitter is connected at the distant end of the other, opposite direction approach section in the same track.

A track circuit receiver unitis coupled to the rails of each approach section by one or more tuned coils, one receiver unit serving both approach sections in each track involved in the crossing arrangement. Each coil is inductively coupled to the shunting rail of the intersecting track at the crossing, that is, the rail of the other track which forms a permanent shunt across the track section for which train detection is desired. A storage or averaging capacitor associated with each receiving coil is charged by the induced or picked-up energy pulses through a full wave rectifier arrangement having a center tap return to the coil. This capacitor is separate from the tuning capacitor connected across the full coil to improve the selectivity of the energy pickup.

from the shunting rail. Each capacitor receiving the rectified voltage from the associated coil provides one input signal to a dual channel, vital level detector device. This level detector includes AND logic circuitry so that an output occurs when and only when a proper level input signal is received by both channels simultaneously. The occupancy of any-part of either approach track section by a train or even a single axle and wheel unit of a car so reduces the level of the oscillating track current pulses flowing through the permanent rail shunt that the input signal is reduced below the predetermined critical or vital level at the level detector portion of the receiver unit. The output of the level detector which is normally amplified by a relay driver arrangement to energize a vital type track relay, becomes insufficient for relay energization thus registering the occupancy condition of both approach track sections. That is, when the relay is energized due to the existence of an output signal from the vital level detector, a nonoccupied condition of the approach sections is registered. At any time that the relay is released, either due to the reduction of the track current by a car shunt in either section or by any circuit fault, an occupied condition of the approach sections is registered for controlling the signal or interlocking arrangement.

I shall now describe a specific embodiment of the features of my invention in connection with two different track circuit arrangements at railroad crossings, and will then point out the novel features of the invention in the appended claims. During the specific description, reference will be made from time to time to the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of a track circuit embodying the features of by invention used to detect the approach of trains from either direction in one track of two intersection tracks crossing at grade.

FIG. 2 is another diagrammatic illustration of a similar track circuit arrangement used for train detection in a track section which intersects two railroad tracks.

In each figure of the drawings, similar reference characters designate similar parts of the apparatus.

Referring now to FIG. 1, across the top is shown, by a conventional two-line symbol, a portion of a railroad track 1 which is intersected by a track 2, only a small portion of which is shown at the center. A 90 crossing is illustrated but actually a crossing having any angle may be similarly protected by the arrangement of my invention. 1 shall describe only the track circuit provided for track 1, that is, for both approaches of track I to the crossing. However, a similar detector track circuit arrangement would also be provided for track 2 to complete a full train detection arrangement for the crossing interlocking control. The portion of track 1 illustrated is isolated electrically from the rest of the stretch of that track by the insulated joints J, shown in each rail at the left and right extremes. Train detection beyond these joints J may be by any known conventional track circuit arrangement and this not involved in the features of my invention. At the actual crossing, each rail of track 2 forms a permanent shunt across the rails of track 1 at the point of intersection. This permanent shunt results from the mechanical structure of the crossing tracks, that is, the crossing frogs whih are formed solid between the intersecting rails, that is, form one piece electrically. Actually the entire crossing square is part of this complete electrical connection, there being no insulation at any point within the four rail pieces forming the crossing square. In addition, each rail of track 1, from each set of insulated joints to the crossing shunt, forms an electrically continuous path with the rails either being continuously welded or connected by conventional bond wires to provide the electrical circuits.

A track circuit transmitter means is connected across the rails at the distant end of each approach track section. As used herein, the term distant means the end of the section remote from the actual crossing or at the point where the insulated joints are used to isolate the corssing rails. This transmitter means includes or is supplied by a high voltage, direct current source shown only conventionally in the drawing. Such a source preferably is a regulated, direct current supply on the order of 200 to 600 volts, depending upon the requirement of the installation, from a vital, that is, a fail-safe voltage regulator. A single such voltage regulator may serve for both transmitter means for a particular track and possibly for other similar transmitters in the crossing installation, depending upon the load and the regulator rating. The particular source at each transmitter is connected across the rails at the insulated joints J through a resistor for limiting the maximum possible current flow and a capacitor. By way of specific example, a source is connected to the rails just inside the joints J source the distant end of the left approach section of track lthrough a limiting resistor R1 and a capacitor Cl. At the distant end of the other approach section of track 1, the souce is connected to the rails through resistor R2 and capacitor C2.

Connected in multiple across each set of leads from source to track, between the resistor and capacitor, are a controlled rectifier and a diode. For example, at the left end, the controlled rectifier CR1 is connected so poled as to pass current flowing from the positive to negative terminal of the source, while the diode D1 is connected across the track leads oppositely poled. The controlled rectifier will normally be of the silicon controlled rectifier type but other solid state switch devices having similar operating characteristics and current ratings may be used in the arrangement. A gating circuit is provided for each controlled rectifier to supply periodic gating pulses at a preselected coding or gating rate. Each gating circuit is shown by a conventional block connected to the gate electrode of the corresponding controlled rectifier. The gate trigger circuit is shown by a conventional block since such gating arrangements are well known in the art and any one of several types may actually be used. For example, a specific type is that using a unijunction transistor with a circuit such as shown in FIG. 4.9 on page 52 of the Controlled Rectifier Manual published by the General Electric Company in 1960 (First Edition). A detailed explanation of such transistor gating circuits is given in Section 4.1 l of this manual, beginning on page 50. Specific controlled rectifier gating circuits using unijunction transistors are shown in FIG. 6.1 on page 75 and in FIG. 7.12 on page 96 of this same manual.

Referring to the transmitter means at the left end of track I, when rectifier CR1 is nonconducting, capacitor Cl obviously charges from the source through resistor R1. Depending upon the charging circuit time constant and the controlled rectifier pulsing or gating rate, capacitor C 1 will charge to a voltage substantially corresponding to that of the source. When rectifier CR1 becomes conducting upon the receipt of a gating or trigger pulse applied to its gate electrode, current flows in a rail loop from capacitor C 1 through rectifier CR1 and including the two rails of the left approach section and the permanent shunt formed by the intersecting rail of track 2. Since the length of the rails of the approach section is relatively short, a. large circulating current flows through this rail path due to the high voltage charge existing on capacitor C 1 at the-beginning of the period. Due to the natural inductance of the rails, which is in series with the capacitance of capacitor C1, the L-C reactance of the rail path causes a ringing of the track current or, in other words, causes the circulating current to be oscillatory in nature. The amplitude of each half cycle of this oscillating current decreases as the initial energy from capacitor C1 dissipates. Diode Dl, connected in parallel with rectifier CR1 but oppositely poled, provides a path for the reverse polarity half cycles of this oscillating current during its existence. The pulses of this oscillating, high level track current occur at the frequency determined by the preselected rate of the gating periods, that is, the frequency of the gate trigger circuit arrangement. Rectifier CR1 turns off with each reverse polarity half cycle of the oscillating track current which flows through diode D1. However,if the gating signal still exists when the next positive half cycle of the oscillating current occurs, rectifier CR1 will again conduct. When the gating signal is not present, the pulse transmission is ended when the negative half cycle terminates since rectifier CRl'is then nonconducting and no path exists for the subsequent positive half cycle of the oscillating current. In actual practice, the gating rate will be preselected on the order of 2 to 6 Hz. However, the rate of oscillation of the track current within a pulse period is not too important, and the number of complete cycles of the cur-- rent .within the pulse will be determined by the length of the gatingsignal and the track circuit parameters.

The operation of the transmitter means supplying pulses at the distant end of the right approach track section is identical with that just described for the transmitter at the left. These pulses flow in the rail loop up to and including the permanent shunt across the right approach section formed by the right-hand rail of track 2 at the crossing. In an alternate form, the transmitter arrangement may be coupled to the rails by a track transformer. Since this is an impedance matching device, it results in a lowering of the pulse frequency or rate possible and also the rail-to-rail maximum voltage of each pulse. If it is a short track section, such as within the actual crossing square or diamond when such must be provided with a detector track circuit, it may possibly be preferable to use this alternate coupling of the transmitter. Normally a separate detection circuit for the actual crossing area is not needed. When the crossing angle is or very close thereto, such as shown, no detection within the square is needed since few, if any, of the known configurations of the trucks of railroad cars can be contained entirely within the roughly 56 inch square size of such crossing.

A single track circuit receiver means or apparatus, comprising at least a level detector and logic circuit element, a relay driver element, and a separate track relay TR, is coupled to the rails at the crossing end of each approach section. Each coupling is a tuned coil and an associated storage capacitor charged through a full wave rectifier arrangement by the coil output. Because the track current which flows during each transmitted track pulse has a high level, it has been found that, by proper selection of the receiver coil impedance, the level of the induced voltage, even after rectification, is sufficient to keep this storage capacitor charged to provide operating voltage to the level detector and logic circuit element. For example, a coupling coil Ll, tuned by a capacitor C5, is positioned in inductive relationship with that portion of the left rail of track 2 which forms the permanent shunt across the rails of the left approach section of track 1. Energy or voltage pulses are then inductively produced in coil L1 in accordance with the track current pulses flowing in the rails of the left approach section, and thus also flowing through the permanent rail shunt. Storage capacitor C3 is then charged by the output of coil L1 developed across the center tap lead and two intermediate tap leads and which is full wave rectified by diodes D3 and D4. The positioning of the intermediate winding taps of coil Ll for the rectifier connections is selected in accordance with the installation requirements and parameters. Similar arrangements exist for the other approach section including a coupling coil L2 tuned by a capacitor C6, the coil being coupled to the shunt formed by the right rail of track 2. Energy produced across the center tap of coil L2 and its two intermediate taps is rectified by diodes D5 and D6 and used to produce a charge on the capacitor C4. Although each track current is pulsating in nature, the capacitors C3 and C4 are selected to have large enough capacitance that they tend to average the level of applied voltage pulses and thus provide a continuous voltage level sufficient to operate the track circuit receiver unit.

The charge existing on either capacitor C3 and C4 is applied to the track circuit receiver unit designated by the dot-dash rectangle in the center of the drawing. Actually, the charge on each of these capacitors is applied across a common and one of the other two input termianls on the level detector and logic circuits element 22 which is part of the track circuit receiver. While it may take other well-known forms, the element 22, shown only by a conventional block, preferably is of the fail-safe type disclosed in my copending application for Letters Patent of the US. Pat., Ser. No. 258,879, filed .lune l, 1972, for A Vital Level Detector and AND Gate Circuit Arrangement Employing a Twin- Tee Oscillator, which application has the same assignee as this present application, now Patent No. 3,793,596 issued February 19, 1974. By selecting capacitors C3 and C4 to each have a large capacitance, it is possible to maintain proper operation of the device of this patent even though the input signals to the capacitors are pulses having a low duty cycle. For easy cross reference, the input leads for element 22 have been designated by the same references, that is, l7, l8, and 19, as the inputs to the apparatus of the single drawing FIGURE of the reference patent.

Reviewing briefly, the vital solid state level detecting an AND .c rquitsof s prior pat mpl y a multistage feedback transistor amplifier, a phase shift RC twin-tee network, and a pair of shunt type voltage regulators. The multistage feedback transistor amplifier includes a common collector input stage and a common emitter output stage. One leg of the RC twintee network formed by a first resistor and a first capacitor is connected to the collector electrode of the common emitter output stage. A second leg, formed by a second resistor and a second capacitor of the RC twintee network, is connected to the base electrode of the common collector input stage. A common third leg of the RC twin-tee network is formedby a third resistor and a third capacitor. Each of the shunt voltage regulators includes a current limiting resistor and a Zener diode connected across individual d.c. inputs. One Zener diode is connected to the third capacitor while the other Zener diode is connected to the third resistor of the RC twin-tee network. Since a Zener diode exhibits a high impedance when it is nonconducting, there is an insufficient amount of regenerative feedback at any frequency to sustain oscillations when the dc. input potentials do not exceed the threshold value of the corresponding Zener diode. However, when the value of the dc. input potentials exceeds the threshold values, the Zener diodes break down and become conductive. The conduction of the Zener diodes is accompanied by a sudden reduction in.the impedance value in the common capacitve and resistive legs of the twin-tee network. The reduced impedance causes a dramatic increase in the amount of regeneration at a predetermined center frequency. Thus, the multistage amplifier produces an output so that a.c. signals or oscillations will be available on the output terminals when and only when'both of the dc. input potentials exceed the predetermined threshold value. lf the track circuit ar-- rangement is applied to only one track section having a permanent rail shunt at only one end, e.g., aspur track section in approach to the switch joining a main track, the level detector element is modified to provide single channel level detection and to eliminate the logic AND circuit.

The outputs of the level detector and logic circuit element 22 appear at leads l6 and 17a, the latter being common with input lead 17. These references are equivalent to the similar references in my aforementioned patent. These output leads are connected so that the output signal is applied to a relay driver el m n Which y be if th M16 Shown in Letters Patent of the US. Pat. No. 3,553,488, issued Jan. 5, 1971, to John O. G. Darrow and Raymond C. Franke, for a Fail-Safe Circuit Arrangement. Specific reference is made to the dashed blocks 4, 6, and 7 of the drawing (FIG. 1) of this reference patent. However, it is to be understood that other types of relay driving circuits may be used in the arrangement of my invention. Generally, the circuit of element 23 requires at least an input amplifier gain stage and an output amplifier stage feeding a rectifier network, such as a direct current voltage doubler, as shown in the reference patent, particularly the specific dashed blocks noted above.

The output of element 23 at leads and 41, which are the same references as those used for the relay driver output in FIG. 1 of the cited Darrow et al patent, is applied to a vital type electromagnetic track relay lTR. This relay is shown by a conventional symbol for vital relays but only a single set of transfer contacts is shown, this being sufficient to illustrate the operation of the relay in the system of my invention. Briefly, this relay operates in a fail-safe manner, and when energized closes its front contact to register a nonoccupied condition for the two approach track sections to the crossing. When deenergized for any reason, this relay positively releases to close a back contact to register an occupied track section condition. It will be noted, of course, that this single track relay functions as the joint occupancy indicator for both crossing approach sections of track 1.

in describing the operation of the arrangement of FIG. 1, it is assumed that, as shown, no train is occupying either of the approach track sections in track 1. Each track transmitter supplies pulses of energy to the rails so that pulses of oscillating track current circulate through the rail loop in each track section. The receiver coils L1 and L2 pick up energy pulses from the associated track rails and their output is rectified to charge capacitors C3 and C4, respectively. With no train shunt in either section, the level of energy received by the coils and thus as a charge by capacitors C3 and C4 is sufiicient as inputs to the track circuit receiver, especially the level detector and logic circuit element 22, to cause proper operation so that the output from the relay driver energizes relay lTR. in other words, a similar and sufficient level of energy is applied to both inputs of element 22 to be detected and to produce an output through relay driver 23 to energize relay lTR. Thusjenergized, relay lTR picks up, closing its from contact to register a nonoccupied indication for the approach sections of track 1.

When a train enters either approach section, the wheel-axle shunt competes with the normal permanent rail shunt and drains away part of the track current which is circulating in the rail loop. The reduced current in the rail shunt at the crossing reduces the voltage induced in the receiver coil and thus the level of the signal applied to the corresponding input of element 22. This level detector and AND circuit element requires a preselected level of equal inputs for proper operation. Since this condition does not now exist, there is no output on lead 16 to be supplied to unit 23. This latter unit, with no input, provides no output and relay lTR is deenergized and releases. The release of relay lTR, closing its back contact, registers an occupied approach track condition. This condition is then used to lock the controls of the interlocking signaling arrangement which includes the railroad grade crossing shown. This occupied track circuit condition will continue to exist as the train moves through the stretch of track 1 shown, for example, from left to right, until the entire train clears the insulated joints J at the right of the drawing. If no part of the train then remains in any portion of the track circuit between the sets of insulated joints 1, the track circuit arrangement will return to its normal operation and a nonoccupied section condition will be registered by relay lTR, thus freeing the interlocking signaling arrangement for further controls or other movements. It is to be remembered that, as previously mentioned, no known railroad car is so constructed that both axles of the conventional two axle truck can be fully contained within the actual crossing square at the center of the drawing. Thus no portion of the train can be lost within the track circuit arrangement between the sets of insulated joints.

It is apparent that a broken rail or a loose track lead will reduce the level of the current flowing in the corresponding section and associated shunt below that required for operation of the level detector. This causes relay ITR to release to register an occupied section, just as when a train shunt occurs. Any failure within a track transmitter arrangement cuts off the track current in that section and relay lTR releases. In addition,

any component failure within the track receiver means,

i.e., coupling, level detector, AND logic, or relay qiiv tz...a hesn iigqslyssit patet ts. results in the assured removal or cancellation of all output and relay lTR thus releases to register an occupied approach track. All such faults or equipment failures thus result in a safe failure condition of the track circuit arrangement.

Referring now to FIG. 2, a different type track circuit installation embodying the features of my invention is illustrated in connection with the stretch of a track 3 which is intersected by two tracks 4 and 5. Although tracks 4 and 5 are illustrated as being parallel, such as a double track main line, it is not necessary that these two tracks be parallel at their intersection of the single track 3. The portion of track 3 to be protected, or in which train detection is to be provided, is bounded at its ends by the two permanent rail shunts formed by the right rail of track 4 and the left rail of track 5. The track circuit includes a single track or pulse transmitter 25 connected to the rails approximately at the mid point of the section and a single track circuit receiver 24 with inputs from receiver coils at each end of the track section.

The track transmitter 25 of FIG. 2 is identical with either of the track transmitters of FIG. 1 and thus is not shown in detail. This transmitter provides periodic pulses of oscillating current to the rails which flow and circulate in each direction from the connections of the transmitter and thus through each pennanent rail shunt at the opposite ends of the track section. The single track circuit receiver arrangement, shown by the conventional block 24, includes such elements as 22 and 23 of FIG. 1. Since the receiver unit was fully described in connection with FIG. 1, it is not necessary to repeat such description or to show the specific sub-elements of the receiver unit. Associated with the receiver are two receiver coils LlA and L2A, each tuned by a capacitor CSA and C6A, respectively. Each such receiver coil is placed in inductive relationship with the corresponding permanent rail shunt at its location. Each of these receiver coils is similar to those shown in FIG. 1 and, when track current is flowing, the output of each receiver coil is rectified through the diode arrangement illustrated and charges an associated storage capacitor C3A or C4A, respectively. The voltage charges on capacitors C3A and C4A are applied to the inputs of the track circuit receiver 24 and specifically to the inputs, previously described, of the level detector and logic circuit element.

With no train shunt within the section of track 3 and thus two equal inputs from storage capacitors to the track circuit receiver, this unit provides an output to energize track relay 3TR, whiceis the same type as the track relay in FIG. I, that is, a vital relay. Relay 3TR is thus picked up to register a nonoccupied section of track 3 between the two rail crossings. When any portion of a train or railing car, that is, a wheel and axle shunt, occupies any part of the section of track 3 between the crossings, the current flow in the permanent rail shunt is reduced in that portion of the track and possibly in both portions so that the output of the corresponding receiver coil is below the preselected level. This causes the receiver unit to fail to produce an output and relay 3TR releases to register an occupied track indication. The nonoccupied or occupied track indication represented by the position of the transfer contact of relay 3TR, as shoTvn, is then used in the control circuits for the interlocking control apparatus governing the movement of trains through the corresponding interlocking including the two grade crossings shown. The operation of the apparatus in FIG. 2 as a train moves through track 3 over both grade crossings in turn will be obvious when taken in connection with the description of the apparatus operation as illustrated in FIG. 1.

It is to be noted that, with the crossings shown in FIG. 2, a car cannot span the section of track 3 with all axles on each truck within the crossing squares. As previously explained, no known railroad car truck has its two axles so spaced as to be contained within the approximately 56 inch square of such 90 crossings. If an acute angle crossing is used because of the position in the tracks so that the rail lengths within the crossing diamond are extended, it may be necessary to provide detection within theactual rail crossing by an arrangement based on the operating principles shown in FIG. It is also to be understood that, in the arrangement of FIG. 2, train detection in the approach sections of track 3 beyond tracks 4 and 5, that is, beyond the left and right limits of the drawing, may be provided by an arrangement equivalent to that shown in FIG. 1. Other modifications of the detection arrangement, of course, would also be possible. 2.

The arrangement of my invention thus provides vital or fail-safe detection of trains occupying sections of a railroad track in approach to a grade crossing with another intersecting track. This detection is provided up to the permanent rail shunt formed by the intersecting rail of the other track. The track circuit source transmits into the rails high voltage energy pulses which produce an oscillating track current of a nature to overcome extra resistance caused by rust or film on the rails. The single receiver apparatus serves each ap proach along a single track and will detect a train presence or absence in a fail-safe manner. The vital circuits of the receiver elements preclude any false nonoccupied indication resulting from an apparatus failure. The resulting track circuit is applicable to any track arrangement or form at a railroad grade crossing or where a permanent shunt is formed across the rails of a track section. In a grade crossing installation, the arrangement leaves no gaps in the detection in which a car or portion of a car may be lost and thus undetected. The track circuit arrangement further achieves these results in an efficient and economical manner.

Although I have herein shown and described but a single form of track circuit embodying the features of my invention, it is to be understood that various changes and modifications therein may be made within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

l. A detector track circuit arrangement, for detecting the presence of any part of a train occupying a railroad track section having a permanent rail shunt formed by an intersecting rail connected between the section rails at one end, comprising in combination,

a. a transmitter means coupled to the section rails at a point distant from said permanent shunt and comprising, I

l. a direct current energy source having a preselected high voltage,

2. a capacitor connected in series with said source across said section rails at the distant point for receiving an energy charge at a predetermined charging rate,

3. a circuit switch normally in an open circuit condition and operable to a closed circuit condition while a gating signal is applied thereto,

4. a source of periodic gating signals having a preselected repetition rate connected for applying such gating signals to said circuit switch,

5. said circuit switch connected between said source and said capacitor for periodically completing, when in its closed circuit condition, an initial discharge circuit path of a predetermined polarity for said capacitor including said section rails, said capacitor and rails forming an oscillatory circuit for discharging the energy stored in said capacitor, and v 6. a unidirectional circuit element connected in multiple with said circuit switch for providing a second circuit path of opposite polarity for said oscillating current during discharge of said capacitor while a gating signal is applied, and

b. receiver means coupled to said section rails in the vicinity of said permanent shunt and responsive to the flow of said oscillating track current for registering a non-occupied indication for said section when and only when said oscillating current pulses flow in said permanent shunt at or above a preselected amplitude level,

1. said receiver means registering an occupied section indication when the amplitude of current pulses flowing in said permanent shunt is reduced below said preselected level by a rail shunt at any other location in said section or by any other cause.

2. A track circuit arrangement as defined in claim 1 in which said receiver means comprises,

a. a receiver coil inductively coupled to said permanent rail shunt and tuned to respond to the oscillating current in said rail shunt for producing voltage pulses at the periodic gating rate,

b. a full wave rectifier arrangement,

c. a capacitor connected by said rectifier arrangement to said receiver coil for storing a voltage signal proportional to the amplitude level of the current pulses flowing in said rail shunt,

d. a vital level detector means connected to receive the voltage signal stored by said storage capacitor and responsive thereto for producing an output signal when and only when the voltage signal represents a track current pulse amplitude equal to or greater than said preselected amplitude level, and

e. an indication registry means coupled for receiving the output signal from said detector means and operable for registering a nonoccupied or occupied condition of said section as an output signal is received or absent, respectively. 3. A track circuit arrangement as defined in claim 2 in which,.

a. said circuit switch is a controlled rectifier device having a normally open anode to cathode circuit path and a gate electrode to which gating signals are applied to enable current to flow through said anode to cathode circuit path when a voltage of proper polarity is applied thereto, b. said unidirectional circuit element is a diode device through which current flows only when the 40 positive potential of an applied voltage is at the anode electrode, c. said receiver coil is tuned by another capacitor connected across the full winding of the coil, and

d. said indication registry means includes,

1. a vital type relay for registering a nonoccupied or occupied indication as the relay winding is energized or deenergized, respectively, and

2. a relay driver means coupled between said level detector means and said relay for receiving said output signal and responsive thereto for energizing said relay winding only when an output signal is present.

4. A track circuit arrangement as defined in claim 1 in which,

a. said track section is one approach section to a railroad grade crossing and said permanent shunt is formed by one rail of the intersecting track, and which further includes,

b. a second transmitter means coupled to the rails at the distant end of the other opposite direction approach track section having a permanent shunt at its crossing end formed by the other rail of said intersecting track, and operable for applying to said other section rails periodic pulses of high voltage energy to transmit through said other section rails oscillating current pulses having the same characteristics as those flowing in said one section rails,

c. said receiver means coupled also to said other section rails in the vicinity of the corresponding permanent shunt and jointly responsive to the current pulses flowing in both permanent shunts for registering a joint nonoccupied indication for both approach sections when and only when the amplitude of oscillating current pulses flowing in each pennanent shunt is at or above said preselected level,

1. said receiver means registering an occupied approach indication when the amplitude of the current pulses flowing in the permanent shunt of either section is reduced below said preselected level by a rail shunt at any other location in the corresponding section or by any other cause.

5. A track circuit arrangement as defined in claim 4 in which said receiver means comprises,

a. a separate receiver coil inductively coupled to each permanent rail shunt and tuned to respond to oscillating current flowing in the associated rail shunt for producing voltage pulses at the periodic gating rate of the corresponding transmitter gating signal source,

b. a full wave rectifier arrangement associated with each receiver coil,

c. a separate capacitor connected to each receiver coil by the associated rectifier arrangement for storing a voltage signal proportional to the amplitude level of the current pulses flowing in the associated rail shunt.

d. a vital level detector and logic means connected to receive the voltage signals stored by both said storage capacitors and jointly responsive to both input signals for producing an output signal when and only when each input signal represents a track current amplitude equal to or larger than said preselected amplitude level, and

e. an indication registry means connected for receiving the output signal from said level detector and logic means and operable for registering a nonoccupied or occupied condition of said approach section as an output signal is present or absent, respectively.

6. A track circuit arrangement as defined in claim 5 in which.

a. each circuit switch is a controlled rectifier device having a normally open anode to cathode circuit d. said indication registry means includes,

1. a vital type relay for registering a nonoccupied or occupied indication as the relay winding is energized or deenergized, respectively, and

2. a relay driver means coupled between said level detector means and said relay. for receiving said output signal and responsive thereto for energizing said relay winding only when an output signal is present. 7. A track circuit arrangement as defined in claim 1 in which,

a. said track section extends beyond the point of cow pling of said transmitter means to said rails to another permanent rail shunt, formed by another intersecting rail, both rail shunts being substantially equidistant from said coupling point of said transmitter means to said section rails,

1. said pulses of oscillating track current are transmitted in both directions through said rails by said transmitter means,

b. said receiver means is also coupled to said section rails in the vicinity of said other permanent shunt and jointly responsive to the current pulses flowing in both permanent shunts for registering a nonoccupied indication for said section when and only when said oscillating current pulses flow in each permanent shunt at or above said preselected amplitude level, I 1. said receiver means registering an occupied section indication when the amplitude of the current pulses flowing in either permanent shunt is reduced below said preselected level by a rail shunt at any other location in said section or by any other cause.

8. A track circuit arrangement as defined in claim 7 in which said receiver means comprises,

a. a separate receiver coil inductively coupled to each permanent rail shunt and tuned to respond to oscillating current flowing in the associated rail shunt for producing voltage pulses at the periodic gating rate of the gating signal source,

b. a full wave rectifier arrangement associated with each receiver coil,

c. a separate capacitor connected to each receiver coil by the associated rectifier arrangement for storing a voltage signal proportional to the amplitude level of the current pulses flowing in the associated rail shunt,

d. a'vital level detector and logic means connected to receive the voltage signals stored by both said storage capacitors and jointly responsive to both input signals for producing an output signal when and only when each input signal represents a track current amplitude equal to or larger than said preselected amplitude level, and

e. an indication registry means connected for receiving the output signal from said level detector and logic means and operable for registering a non0ccupied or occupied condition of said section as an output signal is present or absent, respectively.

9. A track circuit arrangement as defined in claim 8 in which,

a. said circuit switch is a controlled rectifier device having a normally open anode to cathode circuit path and a gate electrode to which gating signals are applied to enable current to flow through said anode to cathode circuit path when a voltage of proper polarity is applied thereto,

b. said unidirectional circuit element is a diode device through which current flows only when the positive potential of an applied voltage is at the anode electrode,

v c. each receiver coil is tuned by another capacitor connected across the full winding of that coil, and

(1. said indication registry means includes,

1. a vital type relay for registering a nonoccupied or occupied indication as the relay winding is energized or deenergized, respectively, and

is present. 

1. A detector track circuit arrangement, for detecting the presence of any part of a train occupying a railroad track section having a permanent rail shunt formed by an intersecting rail connected between the section rails at one end, comprising in combination, a. a transmitter means coupled to the section rails at a point distant from said permanent shunt and comprising,
 1. a direct current energy source having a preselected high voltage,
 2. a capacitor connected in series with said source across said section rails at the distant point for receiving an energy charge at a predetermined charging rate,
 3. a circuit switch normally in an open circuit condition and operable to a closed circuit condition while a gating signal is applied thereto,
 4. a source of periodic gating signals having a preselected repetition rate connected for applying such gating signals to said circuit switch,
 5. said circuit switch connected between said source and said capacitor for periodically completing, when in its closed circuit condition, an initial discharge circuit path of a predetermined polarity for said capacitor including said section rails, said capacitor and rails forming an oscillatory circuit for discharging the energy stored in said capacitor, and
 6. a unidirectional circuit element connected in multiple with said circuit switch for providing a second circuit path of opposite polarity for said oscillating current during discharge of said capacitor while a gating signal is applied, and b. receiver means coupled to said section rails in the vicinity of said permanent shunt and responsive to the flow of said oscillating track current for registering a non-occupied indication for said section when and only when said oscillating current pulses flow in said permanent shunt at or above a preselected amplitude level,
 1. said receiver means registering an occupied section indication when the amplitude of current pulses flowing in said permanent shunt is reduced below said preselected level by a rail shunt at any other location in said section or by any other cause.
 2. a relay driver means coupled between said level detector means and said relay for receiving said output signal and responsive thereto for energizing said relay winding only when an output signal is present.
 2. a relay driver means coupled between said level detector means and said relay for receiving said output signal and responsive thereto for energizing said relay winding only when an output signal is present.
 2. a relay driver means coupled between said level detector means and said relay for receiving said output signal and responsive thereto for energizing said relay winding only when an output signal is present.
 2. A track circuit arrangement as defined in claim 1 in which said receiver means comprises, a. a receiver coil inductively coupled to said permanent rail shunt and tuned to respond to the oscillating current in said rail shunt for producing voltage pulses at the periodic gating rate, b. a full wave rectifier arrangement, c. a capacitor connected by said rectifier arrangement to said receiver coil for storing a voltage signal proportional to the amplitude level of the current pulses flowing in said rail shunt, d. a vital level detector means connected to receive the voltage signal stored by said storage capacitor and responsive thereto for producing an output signal when and only when the voltage signal represents a track current pulse amplitude equal to or greater than said preselected amplitude level, and e. an indication registry means coupled for receiving the output signal from said detector means and operable for registering a nonoccupied or occupied condition of said section as an output signal is received or absent, respectively.
 2. a capacitor connected in series with said source across said section rails at the distant point for receiving an energy charge at a predetermined charging rate,
 3. a circuit switch normally in an open circuit condition and operable to a closed circuit condition while a gating signal is applied thereto,
 3. A track circuit arrangement as defined in claim 2 in which, a. said circuit switch is a controlled rectifier device having a normally open anode to cathode circuit path and a gate electrode to which gating signals are applied to enable current to flow through said anode to cathode circuit path when a voltage of proper polarity is applied thereto, b. said unidirectional circuit element is a diode device through which current flows only when the positive potential of an applied voltage is at the anode electrode, c. said receiver coil is tuned by another capacitor connected across the full winding of the coil, and d. said indication registry means includes,
 4. a source of periodic gating signals having a preselected repetition rate connected for applying such gating signals to said circuit switch,
 4. A track circuit arrangement as defined in claim 1 in which, a. said track section is one approach section to a railroad grade crossing and said permanent shunt is formed by one rail of the intersecting track, and which further includes, b. a second transmitter means coupled to the rails at the distant end of the other opposite direction approach track section having a permanent shunt at its crossing end formed by the other rail of said intersecting track, and operable for applying to said other section rails periodic pulses of high voltage energy to transmit through said other section rails oscillating current pulses having the same characteristics as those flowing in said one seCtion rails, c. said receiver means coupled also to said other section rails in the vicinity of the corresponding permanent shunt and jointly responsive to the current pulses flowing in both permanent shunts for registering a joint nonoccupied indication for both approach sections when and only when the amplitude of oscillating current pulses flowing in each permanent shunt is at or above said preselected level,
 5. A track circuit arrangement as defined in claim 4 in which said receiver means comprises, a. a separate receiver coil inductively coupled to each permanent rail shunt and tuned to respond to oscillating current flowing in the associated rail shunt for producing voltage pulses at the periodic gating rate of the corresponding transmitter gating signal source, b. a full wave rectifier arrangement associated with each receiver coil, c. a separate capacitor connected to each receiver coil by the associated rectifier arrangement for storing a voltage signal proportional to the amplitude level of the current pulses flowing in the associated rail shunt, d. a vital level detector and logic means connected to receive the voltage signals stored by both said storage capacitors and jointly responsive to both input signals for producing an output signal when and only when each input signal represents a track current amplitude equal to or larger than said preselected amplitude level, and e. an indication registry means connected for receiving the output signal from said level detector and logic means and operable for registering a nonoccupied or occupied condition of said approach section as an output signal is present or absent, respectively.
 5. said circuit switch connected between said source and said capacitor for periodically completing, when in its closed circuit condition, an initial discharge circuit path of a predetermined polarity for said capacitor including said section rails, said capacitor and rails forming an oscillatory circuit for discharging the energy stored in said capacitor, and
 6. a unidirectional circuit element connected in multiple with said circuit switch for providing a second circuit path of opposite polarity for said oscillating current during discharge of said capacitor while a gating signal is applied, and b. receiver means coupled to said section rails in the vicinity of said permanent shunt and responsive to the flow of said oscillating track current for registering a non-occupied indication for said section when and only when said oscillating current pulses flow in said permanent shunt at or above a preselected amplitude level,
 6. A track circuit arrangement as defined in claim 5 in which, a. each circuit switch is a controlled rectifier device having a normally open anode to cathode circuit path and a gate electrode to which gating signals are applied to enable current to flow through said anode to cathode circuit path when a voltage of proper polarity is applied thereto, b. each unidirectional circuit element is a diode device through which current flows only when the positive potential of an applied voltage is at the anode electrode, c. each receiver coil is tuned by another capacitor connected across the full winding of that coil, and d. said indication registry means includes,
 7. A track circuit arrangement as defined in claim 1 in which, a. said track section extends beyond the point of coupling of said transmitter means to said rails to another permanent rail shunt, formed by another intersecting rail, both rail shunts being substantially equidistant from said coupling point of said transmitter means to said section rails,
 8. A track circuit arrangement as defined in claim 7 in which said receiver means comprises, a. a separate receiver coil inductively coupled to each permanent rail shunt and tuned to respond to oscillating current flowing in the associated rail shunt for producing voltage pulses at the periodic gating rate of the gating signal source, b. a full wave rectifier arrangement associated with each receiver coil, c. a separate capacitor connected to each receiver coil by the associated rectifier arrangement for storing a voltage signal proportional to the amplitude level of the current pulses flowing in the associated rail shunt, d. a vital level detector and logic means connected to receive the voltage signals stored by both said storage capacitors and jointly responsive to both input signals for producing an output signal when and only when each input signal represents a track current amplitude equal to or larger than said preselected amplitude level, and e. an indication registry means connected for receiving the output signal from said level detector and logic means and operable for registering a nonoccupied or occupied condition of said section as an output signal is present or absent, respectively.
 9. A track circuit arrangement as defined in claim 8 in which, a. said circuit switch is a controlled rectifier device having a normally open anode to cathode circuit path and a gate electrode to which gating signals are applied to enable current to flow through said anode to cathode circuit path when a voltage of proper polarity is applied thereto, b. said unidirectional circuit element is a diode device through which current flows only when the positive potential of an applied voltage is at the anode electrode, c. each receiver coil is tuned by another capacitor connected across the full winding of that coil, and d. said indication registry means includes, 